Examples of automobile wheels include a two-piece wheel having a generally cylindrical wheel rim and a generally disc-shaped wheel disc fitted and welded together. The wheel rim that forms the two-piece automobile wheel in general has bead seat portions for supporting a bead of a tire and flange portions on its both front and back sides and groove-shaped well portions between the front side bead seat portion and back side bead seat portion. Further, a wheel rim is known that has a ledge portion for continuously connecting the back side bead seat portion and the well portions together. Here, the well portions facilitate attachment of the tire in which the bead of the tire is recessed into the well portions when the tire is attached.
Development of engine noise reduction technologies reduced engine noise in automobiles. On the other hand, road noise due to vibration input from the road has been becoming more perceivable while an automobile is running. A suggested cause is cavity resonant noise occurring in a tire-internal space which is tightly closed by the tire and the wheel rim. The cavity resonant noise is produced since vibration input from a road while an automobile is running causes a standing wave with a wavelength of the circumference of the tire-internal space and the standing wave causes air column resonance at particular frequencies (for example, 180 to 240 Hz).
To reduce the cavity resonant noise, an example of a configuration with a plurality of protrusions protruding outward in the radial direction provided on the ledge of the wheel rim in the circumferential direction is proposes in JP-A-2002-187405 (“JP '405”), for example. In such a configuration, the cross-sectional area in the radial direction of the tire-internal space is different between the sections where the protrusions are formed on the ledge portion and the sections where the protrusions are not formed; thus, the cross-sectional area changes in the circumferential direction. This allows the frequency of the standing wave produced in the tire-internal space to change, allows prevention of occurrence of air column resonance, and thus provides an effect of reducing cavity resonant noise.
Further, an example of a configuration with a plurality of protrusions protruding outward in the radial direction provided to portions in the axial direction (front-back direction) of the well portions in the circumferential direction is proposed in JP-A-2004-284495 (“JP '495”). Such a configuration, similarly to JP '405, provides the effect of reducing cavity resonant noise since the area of the longitudinal cross sectional of the tire-internal space changes in the circumferential direction.